Monarch butterflies (Danaus plexippus) reared commercially in North America fail to head south on the annual migration for which the species is famous, researchers from the University of Chicago have found.
Even those bred outdoors or raised indoors under migration-inducing conditions do not join in the annual autumn long distance flight, which sees millions of the insects moving from north-western Canada and the US to overwintering grounds in Mexico.
North American populations of the Monarch have fallen by about 90% over the past two decades due to habitat loss, pesticides, and climate change.
This has drastically impacted their annual migrations. As a result, a popular conservation culture has emerged: commercial breeders sell the butterflies to school children while hobbyists raise wild ones in attempts to salvage the species.
But the new study, published in the journal Proceedings of the National Academy of Sciences, indicates these well-meaning efforts may not be a viable solution.
The butterflies’ unique “migratory syndrome” is complex, involving behavioural, physiological and anatomical characteristics. To test it, Ayse Tenger-Trolander and co-authors measured flight orientation, reproductive status and wing shape, as well as their underlying genetic make-up.
Monarchs breed over successive generations during summer and autumn, then migrate south for winter.
For their first experiment, Tenger-Trolander sourced adult specimens from commercial breeders and raised the offspring of each group over two generations in an outdoor garden, confined in mesh cages while being exposed to natural conditions.
The researchers measured the autumn generation’s flight orientation using a simulator. The butterflies bred from commercial monarchs did not fly in any primary direction, while wild ones captured locally as controls oriented mainly southward.
In a second set of experiments, the team raised the offspring of wild-caught monarchs indoors, attempting to mimic outdoor conditions, but these also showed no dominant flight direction.
Bringing a chrysalis that had been growing outdoors inside just as it was ready to emerge also disrupted migratory behaviour, to the researchers’ surprise, suggesting that even interference following a normal outdoor life cycle affects behaviour.
“I thought there was no way that would matter, but it did,” says Tengler-Trolander.
Markers of the females’ muted reproduction during migration were not altered, however, in the group bred from commercial butterflies. This means that aspects of the migratory syndrome are easily decoupled, meaning that reproductive status is not a useful indicator of migratory behaviour.
Wing shape changed in the commercial butterflies: forewings were rounder, reflecting differences between migratory and non-migratory monarchs found in other locations.
There were several genetic discrepancies in the commercially-bred butterflies that could not be attributed to any other non-migratory species, suggesting the alterations in migration-related traits arose as a result of assisted breeding.
“We can’t point to a single genetic change that did it because there are lots of them,” says senior researcher Marcus Kronforst.
“But we think somewhere buried in the genome are the changes that have broken it.
“It looks like buying monarchs to raise and release doesn’t contribute to the migratory population, and raising them indoors probably won’t, either.”
However, the authors don’t want to deter budding conservationists, because engaging with the butterflies helps people care about them.
“If you want to grow milkweed [the species’ sole food source] in your garden and raise monarchs you find around your house,” Kronforst adds, “just don’t take them inside. If you keep them outdoors, they should be totally fine.”
The findings support concerns by conservation groups and scientists, offering “a window into the complexity – and remarkable fragility – of migration”, the authors write.
“These results are relevant to conservation efforts,” they conclude, “especially as the US Fish and Wildlife Service considers whether to list the North American monarch as a threatened species under the US Endangered Species Act.”
